Miniature inductive devices



T. L. CRAIGE MINIATURE INDUCTIVE DEVICES Aug. 9, Y1960 Filed July 26, 1957 INVENTOR THE'ODORE L. CRA /GE ATTORNEY Unite States Patent O 2,948,871 MINIATURE INDUCTIVE DEVICES Theodore L. Craige, Lynbrook, NY., assignor to United Transformer Corporation, New York, NX., a corporation of New York Filed July 26, 1957, Ser. No. 674,342

1`3 Claims. (Cl. 336-83) The present application isa continuation-in-part of my application Serial No. 539,395, entitled Miniature Inductive Devices, and is assigned to the same assignee as is that application, said application tiled October l0, 1955.

The present invention relates generally to miniature and sub-miniature inductive devices, including inductors, reactors and transformers, and to methods of fabricating these.

It is usual in the art of fabricating inductive devices generally, and transformers in particular, to wind one or more coils on a bobbin of insulating material, and to associate the bobbin with a magnetic core fabricated of laminated or powdered magnetic material. The bobbin must possess sufficient mechanical rigidity to remain undeformed when subjected to stresses, such as 'those created by the windings. In typical miniature and subminiature transformers and inductors, or the like, intended for low voltage applications, as `much as 40% of the `Window 'of the usual core structure may be 'occupied by the bobbin itself. In consequence, conventional cores are larger than l have found to be required 'in the practice of the present invention.

ln accordance with the present invention, the coil of an inductive device is pre-Wound, in a bobbin or 'as a self-supporting coil, and after winding'is placed in association with a magnetic core 'and shell structure Vfabricated of a single thickness of unpowdered magnetic material. The magnetic structure is formed of two or morek conjoinable elements which when placed in conjunction provide an annular container for the'c'oil or coils, magnetically coupling the latter, and also 'provide a 'rigid but light enclosure for the coil or coils. The magnetic structure must possess sutncient mechanical rigidity to remain undeformed when subjected to normal strains and must be economical of fabrication, if it is 'to be commercially feasible. Nevertheless, the structure must not constitute a single closed loop or turn electrically, since such a single vloop or turn is subject to extremely high eddy current losses. To avoid a construction which is equivaient electrically to a single short circuited turn, a radial slot is provided in the elements forming the magnetic structure. The slot provides an egress for the wires of the coil or coils and if suitably dimensioned assures low losses in the magnetic structure and hence a structure of high magnetic eiiiciency.

The magnetic material employed may be sheet material, of nickel-iron alloy and such material has been found to provide desirable magnetic properties and the required structural rigidity, and to permit economy of fabrication.

Small inductive devices, such as transformers, are operated frequently with direct current in one or more windings, which tends to saturate the magnetic circuits of the devices. This is normally compensated for by providing an air gap in the magnetic circuit, which is of precisely selected width. ln inductive devices according to the present invention, an air gap or its equivalent may be 2,948,87 Patented Aug. 9, 1960 ICC introduced in the magnetic circuit of the device by arrang ing the two or more elements which together comprise the magnetic circuit with abutting edges in such physical relation that degree of abutment or extent of gap may be adjusted. The relative physical locations of the two elements, and hence the degree of abutment or gap, may be adjusted until optimum operating conditions are attained for a specific application; and thereafter the relative positions may be fixed permanently, as by incorporating the elements within a mass of potting compound.

The provision of an air gap in the magnetic circuit of any miniature or sub-miniature transformer introduces problems of accurate assembly of conjoined parts, Vbecause the air gap must be extremely small and yet must be accurately dimensioned and duplicatable. It has been found that butt joints in units having laminated cores lead to unduly low circuit reactance, because the gap is great relative to core size. On the other hand, interleav ing of core laminations leads to high saturation effects, and the effective gapsizes are not duplicatable or readily controilable. ln general, accordingly, the mechanical problems involved in providing optimum and economically readily duplicatable core gaps in miniature and subminiature transformers has been heretofore insoluble. The Vease of adjustment of air gaps possible in constructions according to the present invention, on the other hand, leads to optimum designs and to economy of fabrication, and permits higher signal outputs for a given percentage distortion, by a factor of about 50, in a typical case, when compared with transformers having laminated cores and employing either butt joints or interleaved laminations.

More specifically, magnetic structures according to the present invention are preferably fabricated of two or more elements of generally circular cross-section, which may be conjoined on a common axis, and when so conjoined provide internally for inclusion of a pre-Wound coil or coils. The circles of juncture of the elements are then represented by abutting surfaces, one or more of which may be an edge of an element. The extent of overlap of the abutting surfaces or the amount of gap can be adjusted, as by slightly relatively displacing the respective elements, and by so doing the effect of an optimum air gap is introduced, since the reluctance of the magnetic circuit provided by the conjoined elements may be varied.

lt is, accordingly, a broad object of the present invention to provide an inductive device of low eddy current loss, and having provision for inclusion of a readily adjustable air gap in its magnetic structure for compensaturation of the magnetic circuit of the device.

It is another object of the present invention to provide miniature annular core structure of a single thickness of magnetic material, provided with a radial slot for reducing eddy current losses.

A further object of the present invention relates to a miniature annular magnetic shell fabricated of a single thickness of laminated magnetic material, which shall provide an enclosure and a support for a coil and which shall include a radial slot for reducing eddy current losses. ttig Still a further object of the present invention resides in 'the provision of an annular magnetic shell structure fabricated of laminated magnetic material, which shall provide an enclosure and a support for a coil which shall include a radial slot for reducing eddy current losses, and which shall provide a readily adjustable air gap in the direction of the lines of magnetic forces generated by current in the coil.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of ythe following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

Figure .1 is an external view in perspective of an inductive device according to the invention;

Figure 2 is -a view insection takenon line 2-2 of Figure l;

fFigure 3 is a view in section taken on the line 3 3 of Figure l;

Figure 4 is a view in perspective of one half the device of Figures l to 3, inclusive; and

Figures 5 to 10, inclusive, Aare views in perspective of various modifications of the structure Iof Figures l to 4 inclusive, each view being taken on a cut-away half of a complete device, and showing magnetic circuit structure to the exclusion of coil structure.

Referring now more specifically to Figures l to 4 of the accompanying drawings, the reference numeral l denotes a hollow core, of generally cylindrical contiguration, and having a central radial aperture 2, of circular transverse cross-section, extending therethrough. The vcore 1 is fabricated of two similar elements 3 and 4, conjoined `at abutting'edges 5. A radial slot 6 is provided, which extends through the core to the central `aperture 2, so that the core constitutes electrically an open loop or single open turn of conductive material. Mounted internally of the hollow core 1 is a cylindrical pre-wound coil 7, the leads 8 of the windings of which 'can extend internally of the radial aperture 2, via the slot 6, and thence extend outside the confines of the core.

The coil 7 may comprise two or more coupled windings of a miniature transformer, or a single winding of a choke, or reactor. The coils may be wound on or without abobbin, and may be insulated from the core 1 by a layer of shellac, or the like, 10.

In thestructure of Figures 1 to 4, and assuming the windings to be those of a transformer, in particular, the eect of an adjustable air gap may beV provided by relatively shifting the axes of the elements 3 and 4 in directions perpendicular to the axes, so as to vary the overlap of the abutting edges 5, or a gap spacer at the point of abutment can provide the desired gap.

In Figure 4 is illustrated, in perspective, the appearance of the core elements of Figures l to 3, `half of each core element being removed to facilitate visualization. It may be observed that the edges preferably overlap or abut in part only, which results in a narrowing of the magnetic path at the edges 5, and the transfer of some of the magnetic flux from one core element to the other through an air path.

In particular, the radial slot 6 is common to the several disclosed embodiments of the present invention, and is correspondingly identified in the several views.

In the embodiment of any invention illustrated in Figure 5 of the accompanying drawings, the core elements are 12 and 13. Core element '12 consists of the Wall 14, of an axial cylindrical aperture 15, and one end closure wall 16, while the core element 13 consists of the outer cylindrical wall 17 of the core together with another end closure wall 18. The open edge 19 of the closure wall 18 abuts against the outer surface of Wall 14, adjoining its edge 20. 'lhe edge 21 of closure wall 16 abuts against the inner surface of outer Wall 17, adjacent its edge 22. Thereby I provide two air gaps 23 and 24 which may be varied by adjusting the axial rell tion of elements 12 `and 13. If desi-red, paper spacers may be included in the air gap, and attention is invited to radial gap 6.

'In the structure of Figure 6, two cylindrical core elements 30 and 31 are employed, which are generally of circular cross-section taken transversely of the common axis of the core elements, and of U-shaped cross-section taken in a plane through a radius and through the' axis of the cylinders. The core element 30, comprises, accordingly, a base 32, and inner and outer parallel Walls 33 currents.

'35 and inner and outer parallel walls 36 and 37. Radial slot 6 is provided for the core elements 30 and 31, so that these c ore elements, or either of them, shall not constitute a single closed turn 'or loop. The region of overlap of wall 36 with wall 33, or of Wall 37 with wall 34, at which the walls are in physical contact. may be varied in respect to area of overlap, by axial adjust-ment of the elements 30 and 3-1. Thereby, the reluctance of the magnetic structure may be controlled.

In the `arrangement of Figure 7, the core is Inade up of a first cylindrical element 40, of U-shaped cross-section taken in a plane through the axis of the cylinder. The open end of the U is closed by a disc 41. The cylinder and the disc are provided with mating radial slots 6. Effectively adjustable air gaps are provided by sliding the disc over the edges 4S of the cylinder or spacing the disc from the cylinder. Instead of a disc, an annular piece can be employed which fits snugly within the opening of the structure.

It will also be realized that any motion of the disc -41 transversely 4of the axis of cylinder 40, will uncover a portion of an edge 45 of the outer wall 47 of cylinder y40, but that the diametrically opposite portion will not `simultaneously have its gap changed, since the degree of superposition of the disc 41 over the latter portion of the edge is not changed by such motion. Assuming the diameter of the disc to equal the outer diameter of the cylinder, then, yany motion of the disc transversely of the axis of the cylinder will serve to increase the reluctance of the magnetic circuit involved.

In the arrangement of Figure 8, an annular core is fabricated of three elements: a. central hollow tube 50 of 4circular cross section, and two similar units 52, 53 formiing a surrounding shell 51. Each of the units includes an outer cylindrical wall and "an end closure wall 54 vextending at right angles to the outer cylindrical wall. The open edges 55 of the closure walls `abut against the central hollow cylinder 50 adjacent the ends thereof, ywhile the open edges 56 of the outer walls abut against leach other, so that when the elements are assembled they form a closed hollow `annulus, within which may Ybe mounted the coil 'or coils 7.

A radial slot 6 is formed in the annulus, to avoid eddy The reluctance of the magnetic path -may be varied by varying the position of the central tube 50 axially, so as to vary the overlap of the open edge `55 .of the closure walls 54 on the inner tube 50.

The embodiment of Figure 9 is similar to that of Fig- .ure 7, except that the central tube of Figure 7 is replaced by a solid cylinder 60.

The system of Figure l0 is generally similar to that of Figure 7. The edges of the outer wall of Figure 7 are, however, ilared outwardly, as `at 62, yand the inner wall is flared inwardly, as at 63, the cover disc 41 bearing against the flared surfaces. The latter extends parallel :with the cover disc, so that contact therewith may be made over the entire flare. Thereby a Wider range of variations of reluctance can be accomplished than is feasible in the system of Figure 7, since a wider range of displacement is available to the disc 41.

If desired, a gap spacer may be provided between cover disc 41 and the flanges 62 and 63.

In production of units according to the present invention, the coil is pre-wound, and is either self-supported or wound on a bobbin. The elements of a magnetic shell, according to the invention, are placed about the coil and provide a support and an enclosure therefor. The airgap is adjusted for proper operation of the device electrically, and the unit -then clamped to prevent relative movement of the elements. The unit is then impregnated with epoxy resin, which, when cured, provides a rigid structure.

While I have described and illustrated one specic embodiment of my invention, it will be clear that variations of the general arrangement and of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. An inductive device including a magnetic shell, a coil located internally of said shell, said shell including a first annular member, a second annular member, said members being coaxial and in metallic contacting and abutting relation, and arranged to provide an enclosure for said coil, and a radial slot extending through said members in said shell for reducing eddy current losses, said slot being constructed and arranged so that said shell constitutes an open loop circumferentially, said magnetic shell being fabricated of a single thickness of laminate, unpowdered magnetic material.

2. An inductive device including a hollow magnetic shell, said shell including a rst annular member, a second member having axial symmetry with said first annular member, said members being coaxial, at least one of said members being hollow annd including parallel annular edges defining an opening for said at least one of said members, the other of said members being arranged to constitute a closure extending over said opening in metallic contact therewith throughout, and a radial slot extending through said members so arranged that said shell constitutes an open loop when taken circumferentially, the area of abutment and contact of said closure with said parallel annular edges being adjustable and defining an air gap of selective extent by virtue of the extent of said overlap, said shell being fabricated of magnetic unpowdered laminate material.

3. An inductive device including a hollow magnetic shell, said shell including two `conjoined members, each of said members having axial symmetry about a common axis and substantially equal diameters, said members when conjoined providing a substantially closed annular enclosure having a cylindrical inner wall, at least one of said members having two circular walls abutting against and metallically contacting the other of said members, `and a radial slot extending through said members so arranged that said shell constitutes an open loop circumferentially, the area of abutment and contact of said abutting circular walls being adjustable and `defining an air gap of selective extent, said shell being fabricated of magnetic unpowdered laminate material having walls of uniform thickness.

4. The combination according to claim 3 wherein said abutting and contacting circular walls are edges.

5. 'Ihe combination according to claim 3 wherein said abutting walls are axially extending walls.

6. The combination according to claim 3 wherein one of said members is U-shaped in radial section.

7. The combination according to claim 3 -wherein one of said members is a disc.

8. The combination according to claim 3 wherein both said members are U-shaped in radial section.

9. The combination according `to claim 3 wherein both. said members are U-shaped in radial section and have abutting edges.

10. The combination according to claim 3 wherein both said members are U-shaped in radial section and have abutting axially extending walls.

11. The combination according to claim 3, wherein each of said members is L-shaped in radial section.

12. A miniature inductive device having a hollow, substantially closed annular magnetic shell of unpowdered laminate magnetic material, said shell including a plurality of members conjoinable to form said shell, the areas of metallic conjunction of said members being areas of contact and being adjusted to provide a predetermined air gap, and a radial slot extending through said magnetic shell whereby said shell constitutes an open loop circumferentially.

13. An inductive device including a magnetic shell, a coil located internally of said shell, said shell including a rst annular member, a second annular member, said members being coaxial and in abutting relation, and arranged to provide an enclosure for said coil, and a radial slot extending through said members in said shell for reducing eddy current losses, said slot being constructed and arranged such that said shell constitutes an Open loop circumferentially, said magnetic shell being fabricated of a single thickness of laminate, unpowdered magnetic material, wherein said coil includes leads and wherein said leads extend through said slot.

References Cited in the file of this patent UNITED STATES PATENTS 1,287,982 Hartley Dec. 17, 1918 2,498,702 Nahman Feb. 28, 1950 2,628,342 Taylor Feb. 10, 1953 2,677,123 Smoot et al Apr. 27, 1954 2,682,021 Elmen June 22, 1954 2,768,359 Side Oct. 23, 1956 FOREIGN PATENTS 421,353 Great Britain Dec. 13, 1934 

